Robin Roberts, lead anchor on ABC's Good Morning America, tearfully announced Monday that she has been diagnosed with a blood disorder and must undergo a bone marrow transplant. Roberts beat breast cancer five years ago, and this diagnosis is a result of her cancer chemotherapy treatments.

"Today, I want to let you know that I’ve been diagnosed with MDS or myelodysplastic syndrome. It's a disease of the blood and bone marrow and was once known as pre-leukemia," Roberts explained on GMA, surrounded by her co-anchors. "My doctors tell me I'm going to beat this -- and I know it's true."

The 51-year-old anchor will begin chemotherapy Monday in preparation for a bone marrow transplant later this summer. Thankfully, Roberts said, her sister Sally-Ann Roberts, who is an anchorwoman on WWL-TV in New Orleans, will be her donor. "I am very fortunate to have a sister who is an excellent match, and this greatly improves my chances for a cure," she said.

"I have your back, sister," Sally-Ann said, before giving Roberts a kiss on the cheek.

Robin discovered her MDS diagnosis, and underwent a procedure to extract bone marrow, on the day before she was to interview President Barack Obama.  That interview grabbed headlines when the president used it to announce his support of gay marriage.

"The combination of landing the biggest interview of my career and having a drill in my back reminds me that God only gives us what we can handle and that it helps to have a good sense of humor when we run smack into the absurdity of life," she said during Monday's GMA.

Roberts will undergo a bone marrow transplant later this summer.